This invention relates generally to the art of packaging films and more particularly to films useful in the packaging of food and other products.
It is common practice in packaging many goods, including food items, to use what is generally known as thermoforming. In the thermoforming process, a forming web made from a flexible thermoplastic material is advanced to cover a forming die, and heated and drawn down by vacuum into the forming die to form a film cavity. A food or other product to be packaged is inserted into the film cavity, and covered with a non-forming web fed from a second roll stock. The forming and non-forming webs are sealed to form a finished package. The non-forming web is typically referred to as lidstock.
In some applications, instead of forming a bottom web, a preformed tray is used. Polystyrene foam rigid packaging trays are well known. A lidstock is often used in combination with a foam tray to package a food or other item.
Sometimes the foam tray includes a barrier layer to provide longer shelf life to the packaged product. This barrier layer is sometimes supplied by adhering a flexible oxygen barrier film or layer to a polystyrene foam sheet prior to thermoforming into trays, or the oxygen barrier film is adhered to the tray after the tray has been made.
Laminates useful for lidstock applications typically must meet stringent requirements imposed by the particular food or other article to be packaged. The package must be capable of adequately protecting the food item after packaging and through the distribution cycle until the packaged product reaches the end user at point of sale.
The flexible material must also have sufficient abuse resistance to physical and mechanical abuse imposed by the packaging system.
Yet another requirement of packaging material, especially in lidstock applications, is good heat sealability with respect to the tray.
Limited oxygen transmission through the packaging material under both low and high humidity conditions is an essential feature of a packaging material for long-term storage of oxygen sensitive food products. This oxygen barrier feature should maintain the packaged food products for an extended period.
It is often useful to use antifogging agents in lidstock materials in order to reduce the undesirable aesthetic effect of droplets of water on the interior surface of the packaging material. For example, the Cryovac Division of W. R. Grace & Co.--Conn. currently offers-commercially a lidstock laminate having a first substrate and second substrate, in which the sealant layer of the first substrate is a blend of linear low density polyethylene and an antifogging agent. The first substrate also includes an intermediate layer of an EVA/antifogging agent blend, and a bonding layer of EVA. The first substrate is bonded, at its bonding layer, to a second substrate of saran coated polyester. Unfortunately, it has been found that when antifogging agents are present in one or more layers of certain laminates used in lidstock applications, a relatively weak intersubstrate bond is formed when the film (first substrate) containing the antifogging agent is adhered by conventional adhesive lamination to a polyester or saran coated polyester substrate (second substrate). This is found to be true even if the bonding layer of the antifog lidstock material does not itself contain an antifoggging agent, as in the case of the present commercial product. It is believed that the antifogging agent is present in the bonding layer by reason of migration from other layers of the film, and also from skin-to-skin contact of a single wound film roll prior to lamination. To cure this deficiency in the existing product, it has been found necessary to bind the first and second substrates by a separate extrusion lamination step. Low density polyethylene is used as the bonding agent.
It is known to extrude a first film substrate, wind up the extruded film, and later expose one surface of the film to corona discharge just prior to conventional lamination of the substrate to a second substrate. The inventors have found that even with the corona treatment step, only weak intersubstrate bonds can be formed if the first substrate includes an antifogging agent. This is true even if the agent is not originally present in the film layer to be bonded to the second substrate.
The inventors have discovered that the problem of a weak intersubstrate bond can be solved by corona pre-treatment, soon after extrusion, of the bonding layer of a film containing an antifogging agent. This, along with corona treatment during the lamination process, results in greatly improved intersubstrate bond strength between the film and a substrate which is conventionally adhered to the film. The bond strength of the first and second substrate are of course very important in actual end use packaging applications. Delamination of the lidstock material could cause damage to the package contents during distribution and storage of the package, because of loss of oxygen barrier properties. Package appearance could also be adversely affected.
Of interest is U.S. Pat. No. 4,788,105 issued to Mueller et al. which discloses a thermoplastic laminate with two substrates, one of the substrates having polyester and one of the substrates having a layer of LLDPE, and a bonding layer of ethylene vinyl acetate copolymer.
Also of interest is U.S. Pat. No. 5,118,561 issued to Gusavage et al. which discloses a barrier foam tray.
Also of interest is Canadian Patent 1,289,522 (Davis) discussing thermoformed food packaging.
It is an object of the present invention to provide a thermoplastic laminate suitable for the packaging of food and other products.
It is a further object of the present invention to provide a thermoplastic laminate useful as lidstock to be used in combination with foam trays, such as oxygen barrier foam trays.
It is still another object of the present invention to provide a thermoplastic laminate which exhibits improved intersubstrate bond strength.